Polyester has a long history of successful use in packaging,
especially food packaging. Biaxially oriented Mylar polyester films
were introduced in 1952, and the polyester bottle for carbonated
beverages was commercialized in 1978.

Today, the use of amorphous polyester (APET) sheet in thermoforming
is one of the fastest growing segments of packaging. Thermoformed cups,
trays, clamshells, blisters, and organizers are used in a variety of
food packaging applications and for medical products and hardware. Cups
for cold beverage service, especially on airlines, ahd tubs for
delicatessen products are two of the newest and fastest growing uses.
Trays for bakery products are the next likely area of rapid growth. In
these products, APET is replacing crystal polystyrenes and PVC. APET
usage is estimated at 60 to 70 million lbs/yr, and double-digit growth
over the next five years has been forecast.

Polyester Advantages

Polyester's advantages over polyolefin as a packaging material
include its moderate oxygen and water barriers; excellent barrier to
flavors, odors, and most solvents; and good resistance to fats and oils.
Polyester neither absorbs flavors nor imparts off-tastes in food
packaging. APET sheet has excellent optical properties--high gloss,
transparency and sparkle, and very low haze. Exposure to gamma ray irradiation during sterilization has little effect. At dosages up to 5
megarads, there is no yellowing and little loss of physical properties.

Polyester processing is safe because no toxic fumes or gases are
evolved and it contains no chlorinated monomers. It is perceived as an
environmentally friendly plastic and is recyclable. Clean edge trim,
trim press skeletons, and defective parts are easily recycled in the
sheeting operation. Polyester is believed to be one of the cleanest
plastics in use today.

Polyester Disadvantages

Polyester properties that reflect toughness--tear, flex, and impact
strength--may be somewhat less than those of polyolefins. Moisture
barrier is also less but is adequate for many applications in the
thicknesses used. Because it is usually easily crystallized, polyester
is not considered as a heat-sealing material.

Polyester is not normally thought of as a barrier polymer because
its oxygen barrier is much less than that of high barrier polymers such
as PVDC, EVOH, and nitrile. However, when compared to polyolefin, its
oxygen barrier is quite high. Because polyester serves as both the
structural and barrier materials, its use in greater thicknesses makes
the barrier adequate for many products.

Polyester Processing

Homopolymer PET, as produced, has little melt strength and is
difficult to process on conventional three-roll stacked sheet lines and
cast film lines. This difficulty has been reduced by special die
designs and use of a top roll that is smaller than the middle roll, and
by a secondary solid phase polymerization step in the resin
manufacturing process that raises the intrinsic viscosity (IV) and thus
the melt strength of the polymer. However, the high IV resins are more
viscous and require more extruder power or higher barrel temperatures.
The result may be lower throughput rates and lower line speeds as well
as increased power usage.

Many potential APET sheet applications do not require the physical
properties of the high IV resins. The following describes the physical
properties and processing characteristics of a new class of polyesters
developed by Du Pont for improved sheet and cast film production.

HMV Polyesters

Selar PTHMV (high melt viscosity at low shear rate) resins are
polyesters that have been chemically modified to have the high melt
strength and "polyethylenelike" rheology needed for improved
extrusion processing. The three commercial resins are Selar [R] PT
7001, homopolymer polyester; Selar [R] PT 8202, medium copolymer polyester; and Selar [R] PT 8307, high copolymer polyester. Their
thermal properties are shown in the Table.

Other copolymers are under development to provide a range of
melting temperatures and crystallization behaviors that can be used to
tailor structures to meet specific needs in the APET market. All HMV
polyesters comply with U.S. Food and Drug Administration (FDA)
Regulation 21 CFR 177.1630 for food contact.

Rheology

The HMV resins exhibit a more non-Newtonian melt viscosity response
to shear rate than standard homopolymer polyesters. In Fig. 1, the melt
viscosity vs. shear rate curves for standard homopolymer polyesters with
IVs of 0.65, 0.72, and 0.84 are relatively flat while the HMV melt
viscosity curve shows much more shear sensitivity. The unique rheology
of the HMV polyester is quite clear when its melt viscosity curve is
compared with that of the 6.5-MI low-density polyethylene (LDPE) in Fig.
1. The HMV melt viscosity shows a typical temperature dependence (Fig.
2).

The higher viscosities of the high-melt-strength, high-IV standard
polyesters used in sheet extrusion require an increase in extruder drive
power, which causes an increase in extrusion pressure and shear heating.
Because there is little viscosity reduction at high shear in the
extruder (Fig. 1), at a given screw speed, reductions in drive power,
and melt pressure, can be accomplished only by increasing stock
temperature. This, however, decreases melt strength and extruder
output. Figure 1 shows that the HMV polyesters process through the
extruder like a 0.72-IV resin, yet they have the high melt strength
desirable in sheet casting.

The "polyethylene-like" processability of the HMV
polyester can also be seen in the relatively high melt swell as the
polymer leaves the die. Melt swell is related to the viscoelastic properties of the polymer and contributes to the improved processability
in film and sheet. Melt swell was measured by extruding a strand at a
shear rate of 117 [sec.sup.-1] from a Kayeness rheometer into a buoyant
solution and then comparing the diameter of the strand with the diameter
of the capillary. The results given below clearly show the melt swell
of HMV polyester to be more like LDPE than conventional polyesters.

The crystallization behavior of the resin is important in APET
sheet production and thermoforming. The crystallization rate will
determine the operating temperature of the three-roll stack and may
affect the amount of amorphous regrind that can be processed through the
drying hopper. Crystallinity in APET sheet causes hae and makes
thermoforming difficult or impossible. However, in drying regrind,
rapid crystallization is desirable to prevent agglomeration or
"caking" in the drying hopper.

Homopolymers have a higher crystallization rate than copolymers.
The type and amount of copolymer content and other components determine
the rate of crystallization. HMV polyesters offer the range of
crystallinity behavior needed for a variety of APET applications.
Crystallization times at 135 [degrees] C are shown below.

The extruder conditions for the HMV polyesters are much the same as
with other APET resins used for sheet but less demanding than those
required for PETG, another amorphous polymer. They are easily processed
on a w ide variety of extruders. An extruder with an L/D of 30:1 is
preferred, to provide more melting time and a more uniform melt
temperature. However, extruders with L/Ds ranging from 24:1 to 32:1
have been used successfully.

For optimum performance, the screw should be designed for the
rheology and melting characteristics of the HMV polymer and the desired
throughput from the particular extruder. Because of the
polyethylene-like processing characteristics of the HMV polymers, good
results have been obtained using a general-purpose screw with a 3:1
compression ratio. HMV polyesters also run well on double-flighted
screws designed for PETG and on polystyrene screws.

The extruder barrel temperature and downstream equipment
temperatures should be optimized for each extrusion system. Screw
cooling is not required but may be used if available. Reversed
temperature profiles are usually used for polyesters. The feed zone
temperature depends on the melting point of the resin and the incoming
material temperature. The temperature profile for a typical 4.5-in,
30:1-L/D extruder with a barrier-flighted polyester screw is: feed zone
290 [degrees] C, 285 [degrees] C, 280 [degrees] C,275 [degrees] C, 270
[degrees] C, with all downstream equipment at 270 [degrees] C. A melt
temperature range of 270 [degrees] C to 280 [degrees] C is normal.

A gear pump and melt mixer feeding a coathanger-type flex lip die
is suggested for best sheet gage uniformity.

Drying and Recycling

As with all polyesters, HMV resins must be properly dried before
extrusion to prevent the hydrolytic degradation that results in loss of
melt strength and physical properties of the sheet. HMV polyesters are
supplied as fully crystallized pellets, which permits drying at high
temperatures without bridging or agglomerating in the drying hopper. An
in-line, dehumidifying dryer system with an insulated hopper is
recommended. Typical drying conditions are 165 [degrees] C air at a -30
[degrees] C dew point with an air volume of 1 cfm/lb per hour of resin
usage. HMV polyesters, like most others, should be dried to <0.01%
moisture content and preferably to 0.005%.

If properly dried, HMV polyesters are easily recycled in sheet
operations with littles loss of IV or melt strength. In a typical
operation, 20% to 40% amorphous regrind can be blended with virgin resin
pellets and successfully crystallized and dried at 150 [degrees] C.
Higher levels of amorphous regrind should be evaluated carefully to
prevent "caking" or a "melt down" in the drying
hopper.

Sheet Production

Standard polyesters with an IV of 0.80 or greater have sufficient
melt strength for sheet extrusion. Standard homopolymer PETs with IVs
of 0.75 or less tend to pour out of the die like water and sag
excessively. Figure 1 shows that HMV polymers have a melt strength
equivalent to that of standard homopolymer PETs in the 0.80- to 0.90-IV
range. The slope of the HMV melt viscosity curve and its relatively
high melt swell indicate a polyethylene-like rheology.

These properties of the HMV polymers allow a higher drawdown ratio
and production of thinner sheet than is possible with 0.80-IV
polyesters. In one case, the lower limit for nip polished sheet had
been 10 mils. With PT 7001 HMV polyester, 8-mil sheet was easily
produced by reducing extruder speed. Even thinner 6-mil sheet was
produced by increasing the line speed to the maximum.

Coextruded Sheet

HMV APET resins have created new possibilities for the sheet
producer. The range of melt temperatures available in the HMV polyester
family allows coextrusion with heat-sensitive EVOH polymers and Bynel
[R] coextrudable adhesive resins to produce a five-layer, high-barrier,
transparent, thermoformable APET-based sheet. Five-layer sheet has been
made with a 0.75-mil EVOH layer and a total thickness of 11 mils.

In heat-sealing applications, a two-layer coextruded sheet can be
made using the 7001 homopolymer for the structural layer and the lower
melting 8307 copolymer for the sealing layer.

Thermoforming

The HMV APET sheet is easily thermoformed using vacuum, pressure,
and plug-assist methods. Sheet made from 8202 copolymer forms well at
the same conditions used for PETG, while sheet made from 7001
homopolymer may require slightly higher over temperatures. The high
melt strength of the HMV resins reduces the amount of sheet sag during
the heating phase. It also helps in producting uniform part thickness
in high-aspect-ratio forming. Cylindrical parts with a
depth-to-diameter ratio of 3:1 have been easily formed.

Sheet temperatures for best forming lie in the 90 [degrees] C to
120 [degrees] C range (195 [degrees] F to 250 [degrees] F). As with
other APET sheet, moderate heat settings over a longer length give
better results than high heats over short lengths.

Mold temperatures of about 35 [degrees] C to 50 [degrees] C (95
[degrees] F to 120 [degrees] F help prevent "markoff" or cold
draw marks in the side walls of formed parts, especially in deep,
vertical-walled parts. The forming cycle time for HMV APET sheet is
about the same as for other APETs and oriented polystyrene, but much
shorter than for polyvinyl chloride sheet.

Cutting e quipment for APET should be well maintained and sharp.
Steel rule dies should be made from a good grade of steel. Aluminum
striker plates are not recommended. Punch and die sets should be tight
fitting and heavy enough to prevent flexing to avoid "angel
hair" and ragged cutting. A progressive or scissortype cut is
preferred.

COPYRIGHT 1991 Society of Plastics Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.